File Format for an Animation
This question is very broad and has no definite answer.

With that being said, I'll write my answer as H.264. Why?

Among others, it's one of the most universal. High compression combined with high quality, especially compared to the compression ratio. I personally use it for most of my video renderings. 

It's also broadly supported, and will run on all major platforms; Windows, Mac, browsers, iPhone/iPad, Android devices, Windows devices, etc.

If you decide, you can easily convert the video clip to MP4 at a later time if you so desire.

It's also one of the industry standards along with MP4, WAV, OGG and more.

So again, this is more of a personal answer on a broad question.

Hope this helps.